Skin for doubly curved sandwichtype panels



June 29, 1954 S. H. HAHN SKIN FOR DOUBLY CURVED SANDWICH-TYPE PANELSFiled Feb. 23. 1952 IN V EN TOR. Stuart H Hahn ATTORA/E YS Patented June29, 1954 S KIN FOR DOUBLY CURVED SANDWICH- TYPE PANELS Stuart H. Hahn,Los Gatos, Calif., assignor to Dalmo Victor Company, San Carlos, Calif.,a corporation of California Application February 23, 1952, Serial No.272,951

3 Claims.

This invention relates to a doubly curved product and to a method offorming the same.

There is very little difficulty encountered in producing a so-calledsingly curved product, such as a segment of a cylinder, since alaminated article can be readily fabricated by providing a suitable coreand facing the same with a sheet on either side thereof. It is obviousthat a flat thin sheet readily conforms to a cylindrical shape. However,when producing a doublycurved product, such as a segment of a sphere ora paraboloid etc, difficulties are immediately encountered in trying toprovide the doubly curved facings or skins, since it is equally apparentthat a flat sheet does not readily con-form to a spherical or otherdoubly curved surface.

In an attempt to circumvent the problem of providing doubly curvedskins, some doubly curved articles have been cast. These cast articleshave been satisfactory where weight is not a problem and Where a highstiffness to weight ratio is not required. However, certain doublycurved articles, such as doubly curved radar reflectors, must be bothlight in weight and havev a high stiffness to weight ratio. In anattempt to lighten cast articles. they have been perforated butunfortunately this produces a relatively flexible product. Flexing ofthese reflectors during use obviously distorts the beam transmitted andthe signals received by these reflectors.

In order to provide a light stiff doubly curved reflector, it has beenthe ractice in the prior art to form a" laminated or sandwich-typearticle having a cellular core and-front and back sheet metal skins.However, unless the core material has a varying thickness from thecenter outwardly, it is necessary to use two sets of very expensive dieequipment to accurately form the concave and convex skins, it beingobvious that both skins cannot be formed by the same die equipment. Ifthe core is made of variable thickness so that the front and rear skinscan be formed by the same die equipment, the die equipment requirementsare reduced .but a high cost is encountered in forming such a variablethickness core.

. It is therefore a main object of the present invention to provide adoubly curved sandwiched article, particularly one having a metallicsur- I face for use as a radar reflector, which article,

although being very light in weight, has a high stiffness to weightratio.

Another main object of the present invention is to provide a method forforming an article having a doubly curved surface, which method issimple and does not necessitate the use of expensive and complexequipment.

Another object of the present invention is .to provide a simple andimproved metal faced radar dish having accuracy of contour and a highstiffness to weight ratio.

Various other objects of the invention will be apparent from thefollowing description taken in connection with the accompanying drawingswherein:

Fig. 1 is a three-dimensional view of a radar dish or reflectorembodying the concepts of the present invention.

Fig. 2 is a front elevational view of the dish disclosed in Fig. 1. i

Fig. 3 is an enlarged cross-sectional view'of the dish taken on line 33of Fig. 2.

Referring to the accompanying drawings wherein similar referencecharacters indicate similar parts throughout, the radar dish orreflector shown and representing one embodiment of my inventioncomprises a doubly curved cellular core 9 having a front facing or skinH secured to the concave side thereof and a rear facing or skin l3secured to the convex side thereof. Since the facings are identical inconstruction, only the front facing ll need be described in detail.

Front facing ll comprises two layers [5 and l1 joined together by anadhesive material, such material throughout the construction beinggenerally entitled l9. Each of the layers [5 andl1 comprises a pluralityof narrow metallic foil strips 2! arranged and adhesively secured inoverlapping or shingled relation to one another. Layer I! is adhesivelysecured to the front concave surface of the cellular core 9 and layers15 and I? are adhesively secured to one another.

As is apparent from Fig. 2, the strips of one layer of facing I l arearranged at an angle, preferably 90, to the otherlayer of the facing toachieve a maximum amount of strength and stiffness. The strips 2| ofrear facing i 3 are similarly arranged and adhesively secured in placeto the rear convex surf-ace of cellular core 9.

It is pointed out that by making the strips 2| thin and relativelynarrow, the amount of wrinkling of the strips is kept to a minimum.These strips can be built up on the core without any appreciablewrinkling if at least two of their I dimensions are small in comparisonto the curvature of the cellular core. The two dimensions selected arethe thickness of the strip and its width. Although, of course, somewrinkling'does occur, the extent thereof is not suflicient todetrimentally efiect the transmission of signals from or the receptionof signals by the dish, since it is the general con-tour of the dish(apart from severe wrinkling) that determines to a large degree whetherreception and transmission of radio frequency energy by theradarrefiector will be distorted or not.

By using thin foil-like forms of light metals, such as aluminum, maximumadvantage can be taken of the hardness and strength which are inherentin severely cold-worked metal, whereas a comparable single-ply sheet ofmetal would necessarily be much softer and weaker.

A novel and inexpensive manner of forming a doubly-curved article asabove described comprises the steps of forming a mal form havin a doublycurved convex workin surface conforming to the curvature desired of theconcave side of the final product. Such a form can be inexpensivelyconstructed of wood. After the provision of such a form, strips 2| oflayer I5 are then laid on the form and overlapped in a shingle fashion,it being preferable that the overlap be approximately 50 per cent of thewidth of a strip so that each layer has in effect an even two-plthickness, therebydistinctly lessening the irregularities on the facesof the dish which would otherwise be produced with a lesser or greateramount of overlapping at the overlapped joints of the strips of the twolayers. Concurrent with the laying of. the strips, a suitable adhesivematerial is applied to th strips so that the strips are secured to oneanother. A suitable adhesive for this purpose is a phenolicthermo-setting resin or an epichlorhydrin polymer, commercially giventhe name of Epon VI. The latter adhesive has a low curing temperatureand low shrinking characteristics upon polymerization. A primaryadvantage of such an adhesive is that all differential expansiondifficulties inherent. in the use of the usual adhesives, which havehigh curing temperatures, is avoided.

Strips 2| of layer I! are next applied or built upon the wooden form ontop of layer i5 and adhesively secured thereto and. are adhesivelysecured to one another in shingled fashion and arranged at an angle,preferably 90, to the strips of layer [5 to achieve maximum strengthfrom the strip construction. Next, a cellular core of uniform ornon-uniform thickness, which may be previously formed doubly curved, byany simple process, is applied and adhesively secured to layer i l, theadhesive. material assuring good conformity of the core to the layer.Such a core can be readily formed from a fiberglass honeycombed panel bya simple heating step or, if the cellular core is to be a metallichoneycombed panel, such can be made up of triangular se ments bent to adoubly curved configuration.

Finally, the two layers of facing I3 are laid and adhesively secured toone another and to the core 0. The adhesive material is then allowed todry or polymerize. However, it may be desired to insure an accurateconformity of the built-up structure to the form and this can be readilyaccomplished and insured by enclosing the built-up structure and theform in a bag and then evacuating the air from the bag to applyatmospheric pressure to the laminated or sandwiched product and force itagainst the curvature of the form.

It is preferable that a separant such as a soap, grease or a commercialmold lubricant be applied to the form prior to building up the layerstherethe resulting article being from on to enable eas removal of thecompleted product.

In the above-described process and in the previously described product,only two cross layers per facing were described, but it is obvious thatas many layers as desired can be added. In fact, it is contemplated thata multiplicity of plied-up layers of strips will be utilized in formingthe usual product.

By way of example and not by way of limitation, foil used in the aboveprocess for producing a dish 6 feet in diameter had a thickness of from.0005 inch to .005 inch and a width approximately 4 inches, the overallthickness of A2 inch to 3 inches thick, Ordinarily, at least two layersper facing will be used so that one layer can cross the other layer.However, in special circumstances, it is contemplated that only onelayer might be used although it is obvious that a twolayer facing willhave considerably more strength than a single layer even taken inproportion to the thickness of the two facings.

Although the building up process. has been described with reference tothe use of strips, it is contemplated that such a construction couldbebuilt up by the use of individual shingles or fishscale-like pieces,although it is believed that the strip formation is less expensive andmore readily carried out than a fishscale-type construction.

It is additionally pointed out that metallic bonding of the individualfoil strips to one another is found unnecessary in the case of radarreflectors becauseof the inherent high capacitative coupling of theclosely adjacent foil pieces which are separated by a few thousandths ofan inch or less by the adhesive bonding material which is normally adielectric. However, conducting or semi-conducting adhesives may be usedas well.

By the present invention a doubly curved sandwich-type article has beenprovided having an extremely high stiffness to Weight ratio and whichcan be easily fabricated w th a minimum use of equipment and time, yetwhich article can be produced having an accurate contour suitable foruse as a radar reflector. Obviously, the invention has wide applicationin the construction of a Wide range of doubly curved articles, bothmetal faced and otherwise, and that the reference to a radar reflectormerely illustrates a practical application of the concepts of thepresent invention.

While I have shown the preferred form of my invention, it is to beunderstood that various changes may be made in its construction by thoseskilled in the art without departing from the spirit of the invention asdefined in the appended claims.

Having thus described my invention, what I claim and desire to secure byLetters Patent is:

1. A doubly curved article having a doubly curved honeycomb core with afacing on each of its opposite sides, each facing having a first andsecond layer, each layer including a plurality of thin strips arrangedand secured in shingled fashion to one another with a constant width ofexposed surface throughout the length of each strip, the innermostlayers of the facings being secured to the honeycomb core, the strips ofeach layer running at an angle to the strips of the immediately adjacentlayer.

2. A doubly curved radar reflector comprising a central honeycombedcore, and interior and exterior metallic skins covering said core, eachof 5 said skins having a first layer of thin foil strips adhesivelysecured to said core, said strips being adhesively secured to oneanother in overlapping shingled relation with a constant width ofexposed surface throughout the length of each strip, a second layer ofthin foil strips adhesively secured to each first layer and arranged atan angle to the strips of the first layer, the strips of each secondlayer being adhesively secured to one another in overlapping shingledrelation with a constant width of exposed surface throughout the lengthof each strip.

3. The method of making a doubly curved article comprising providing amale form having a doubly curved convex working surface; laying a layerof strips of constant width across the convex surface, arranging andsecuring the strips to one another in overlapping shingled fashion witha constant degree of overlap; laying a second layer of strips ofconstant width across the first layer of strips, adhesively securing thestrips of the second layer to the first layer at an angle to the stripsof the first layer, arranging the strips of the second layer andsecuring them to one another in overlapping shingled fashion with aconstant degree of overlap, laying a honeycombed cellular core onto thesecond layer and securing the core to said second layer, laying a thirdlayer of strips of constant width onto the cellular core with the stripsbeing arranged and secured to one another in overlapping shingledrelation with a constant degree of overlap, and laying a fourth layer ofstrips of constant width across the third layer of strips with thestrips of the fourth layer being adhesively secured to the third layerat an angle to the strips of the third layer, arranging the strips ofthe fourth layer in parallel relation and securing them to one anotherin overlapping shingled fashion with a constant degree of overlap.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,448,114 Olson et al. Aug. 31, 1948 2,578,781 Brundige Dec.18, 1951 2,592,080 Thompson Apr. 8, 1952 OTHER REFERENCES 30 Foot Tower,Modern Plastics, December 1950, page 138.

Honeycomb Sandwich Construction, Plastics (London), October 1949, pages64-66.

1. A DOUBLY CURVED ARTICLE HAVING A DOUBLY CURVED HONEYCOMB CORE WITH AFACING ON EACH OF ITS OPPOSITE SIDES, EACH FACING HAVING A FIRST ANDSECOND LAYER, EACH LAYER INCLUDING A PLURALITY OF THIN STRIPS ARRANGEDAND SECURED IN SHINGLED FASHION TO ONE ANOTHER WITH A CONSTANT WIDTH OFEXPOSED SURFACE THROUGHOUT THE LENGTH OF EACH STRIP, THE INNERMOSTLAYERS OF THE FACINGS BEING SECURED TO THE HONEYCOMB CORE, THE STRIPS OFEACH LAYER RUNNING AT AN ANGLE TO THE STRIPS OF THE IMMEDIATELY ADJACENTLAYER.